AFTER several years of effort based on the application of the principles of neutron capture therapy, we have empirically evolved a procedure which now can be added to the few effective treatments for established invasive transplantable neoplasms of animals. Previously, we discussed the biological utilization of thermal neutrons to generate alpha particles and lithium atom fragments from boron-10. We noted the pathways of these fragments are of cellular dimensions1–5, and how, in principle, one might thereby attain sharply localized intense effects. Our present observations on 611 neoplasms may serve to point towards achievement of these goals. In this communication we report the thermal neutron flux which must be incident on the skin over solid neoplasms 8 mm–17 mm in diameter to result in permanent regression after giving sodium pentaborate intravenously in a dose equivalent to 35 µg/g body weight of boron-10. Only a single treatment was required by this procedure.